Fuel cells convert a fuel into usable electricity via chemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
One form of fuel cell component failure involves membrane thinning and eventual rupture, while another involves seal failure; both of these are common in stack end-of-life conditions. Such failure can lead to leakage in general, and in particular to the large and unintended transfer of H2 from the anode portion, side or subsystem within the stack to either the cathode portion, side or subsystem, or overboard altogether. Of these, it is of particular concern when such leakage leads to a high H2 concentration in the presence of air within the cathode portion of the cell or stack.
One remedial approach is to continue to run the stack normally, where the leakage flow is accounted for by introducing additional dilution air into the system. There are limits to the effectiveness of such an approach, especially for larger leakages, as the size of the leakage may exceed the capacity of the system to deliver full dilution air. Another presently-employed remedial action for such drastic failure is known as a “quick-stop”, where the H2 supply is cut off along with disengagement of the load (for example, an electric motor or the like) from the stack while leaving H2 in anode. Typically, a significant shortcoming with the quick-stop approach is that it leaves the H2 in the stack after leak has been detected. Moreover, such an approach is only appropriate if the exhaust emissions exceed a certain threshold (for example, 8%) when full dilution air is accounted for. Although this approach may be good for many failure modes (such as reactant starvation) to prolong stack life, its effectiveness is severely curtailed after a large leak between the anode and cathode flowpaths has occurred. Significantly, most fuel cell systems operate such that the anode-side pressure of the stack is biased over that of the cathode side (often between roughly 1.5 psi (10 kPa) and 3 psi (20 kPa)); in such circumstances, a large leak would lead to significant H2 flow into (and concomitant increased concentration of) a highly localized spot within the stack's O2-containing cathode side. As such, to the extent that the quick-stop approach may be used in certain operational conditions, it is not suitable for shutting down an operating fuel cell system as a way to protect against the fuel (i.e., H2) corruption of the air (i.e., O2) side or subsystem after the formation of a large leak.